Orthogonal frequency division multiplexing (OFDM) has been attracting an increasing interest in various broadband digital communication systems for more than 30 years, with applications now including digital audio/video broadcasting (DAB/DVB), high speed digital subscriber line (DSL) twisted pair, digital cable TV systems and local mobile wireless networks, among many others. In an OFDM system, the whole available frequency band is divided into N subchannels. Each subchannel is used to transmit an independently modulated subcarrier. Existing systems employ two-dimensional (2-D) digital modulation formats, mainly M-ary quadrature amplitude modulation (M-QAM) as well as M-ary phase-shift keying (M-PSK) for subcarrier modulation in various OFDM systems. To maintain orthogonality among subcarriers, a minimum frequency separation between adjacent subcarriers is 1/T, T being the period of an OFDM symbol. An inverse discrete Fourier transform (IDFT), often implemented by an inverse fast Fourier transform (IFFT) algorithm, and a discrete Fourier transform (DFT), often implemented by a fast Fourier transform (FFT), provide an efficient method for implementing orthogonal frequency division multiplexing and demultiplexing in an OFDM system. An OFDM signal with M-ary QAM for subcarrier modulation is often referred to as M-QAM-OFDM.
In a recent article by Fuqin Xiong, “M-ary amplitude shift keying OFDM system”, IEEE Trans. on Commun., vol. 51, no. 10, pp. 1638-1642, October 2003, M-ASK-OFDM (OFDM using M-ary amplitude-shift keying (ASK) for subcarrier modulation) was proposed. Although an M-ary ASK symbol carries only half the number of bits as an M2-ary QAM symbol, M-ASK-OFDM has a bandwidth efficiency equivalent to that of M2-QAM-OFDM, because the minimum frequency separation between subcarriers in ASK-OFDM reduces to ½T, resulting in the number of subcarriers being doubled to 2N. Although what is disclosed by Xiong is correct in principle, some critical problems of implementation are not addressed in the paper.